Experiments on the Three-Layer Richtmyer Meshkov Instability

Abstract

The Richtmyer-Meshkov fluid instability (RMI) can be considered a particular case of the broader Rayleigh-Taylor instability (RTI), in the situation that the fluid interface is impulsively accelerated, as is the case when such an instability is impacted by a shock wave. The study of RMI has significant applications to the research of the Internal Confinement Fusion (ICF) method of nuclear fusion, which involves the superheating of fuel contained within a capsule consisting of multiple, closely spaced layers of material each affected by the formation of such instabilities under such conditions. One important quantity studied in RMI applications is that of the growth in amplitude of the instability structures as they develop from an initial sinusoidal perturbation at the interface, and this property is the main focus of the experiments conducted for this study. The effect of placing a secondary, unperturbed interface just above the well-studied single-interface configuration is studied for its effect on the amplitude growth in the nonlinear regime of RMI. The effects of the presence of this secondary interface are considered with two different gas combinations all of varying density, with a third gas added for a second interface and studied alongside the results of the one-interface case. The gases are vertically stratified in a shock tube with the lightest gas entering at the top, the heaviest gas at the bottom, and the middle layer gas emitted through porous metal plates near where the interface is formed. Experiments are visualized by illuminating one gas seeded with particles with a light sheet from a pulsed laser, with recordings captured by a single high-speed video camera. Amplitudes are measured by defining the interface position at each frame by its maximum brightness gradient and finding its maximum vertical span. The data suggests that the presence of the second, unperturbed interface causes a decrease in amplitude growth during the nonlinear regime of the instability development. Continued research is proposed to explore the accuracy of and reasons for the observations made.